Exhaust apparatuses including exhaust pipes connected to small-sized engines and box-shaped, compact mufflers attached to distal end portions of the exhaust pipes are proposed in, for example, JP-A-11-13452.
FIG. 11 hereof shows an exhaust apparatus disclosed in the above publication. In FIG. 11, the exhaust apparatus, designated by reference numeral 230, includes an exhaust pipe 232 extending from an engine 231, a box-shaped muffler 233 attached to the exhaust pipe 232, and a spark arrester 234 attached to the muffler 233 for preventing sparks contained in exhaust gas of high temperature from being emitted into the air.
The muffler 233 includes a first expansion chamber 235 communicating with the exhaust pipe 232, a second expansion chamber 237 communicating with the first expansion chamber 235 via exhaust passages 241 formed in a partition plate 236, and a tail pipe 238 disposed within the second expansion chamber 237. An exhaust gas flows from the first expansion chamber 235 through the exhaust passages 241 into the second expansion chamber 237. The exhaust gas then flows from the second expansion chamber 237 into the tail pipe 238 through exhaust holes 242 formed in the tail pipe 238.
If the exhaust passages 241 are narrowed or the number of the exhaust holes 242 is reduced for improved exhaust noise reduction, a back pressure increases to thereby heat the muffler 233 to a high temperature. Thus, it is impractical to narrow the exhaust passages 241 or reduce the number of the exhaust holes 242 for improved exhaust noise reduction. If the exhaust passages 241 are widened or the number of the exhaust holes 242 is increased, the back pressure can be reduced to thereby prevent the muffler 233 from being heated too much, but sufficient exhaust noise reduction can not be achieved.
Thus, there has been a demand for an exhaust apparatus for a small-sized engine, which is capable of reducing an exhaust noise with sufficiency while keeping to a minimum a degree to which a muffler is heated.